As land has been used over the years, certain contaminants have been introduced resulting in toxic conditions. In the past, such conditions have been routinely ignored. However, individuals as well as government agencies have become increasingly sensitive to problems which ensue as a result of the presence of hazardous waste including contaminated soils and there is now a general recognition that remediation is not a mere optional expedient.
One such recognized class of contaminants consists of heavy cationic metals such as lead. Capillary and hydraulic flows of water in porous media contaminated by such heavy metal species has resulted in, for example, aquifer contamination. Removal of heavy metals from contaminated soils is energy intensive and time consuming since the mobility of heavy metal ions is orders of magnitude slower in soil than in water. As such, ways have been sought to remediate soil by chemical treatment such as by binding the metal contaminants in place so that they will not leach out of the soil thus producing significant environmental benefits at substantial cost savings.
A common cationic metal requiring remediation in solid waste is lead. Lead as a contaminant is often found in the soils around lead smelters, battery breaking/recycling facilities, incinerator ash facilities and foundries including metal and leaded gasoline manufacturing plants. Contamination occurs when lead containing chemicals are used in the plants, and waste containing the lead is allowed to spill over or drain into the soil. Many abandoned hazardous waste sites are heavily contaminated with lead, threatening human health, the food chain, the ecosystem and the environment. Federal legislation, such as the National Contingency Plan (NCP), the Superfund Act (CERCLA) and the Superfund Amendments Reauthorization Act (SARA) specify the remediation of sites containing lead-toxic soils and other solid wastes.
The Resource Conservation and Recovery Act of 1976, commonly known as RCRA, provided for federal classification of hazardous waste. The statutory language defines "hazardous waste" as solid waste or combinations of solid waste which pose a "substantial present or potential hazard . . . when improperly treated, stored, transported, or disposed of, or otherwise mismanaged." Any solid waste that exhibits one of the hazard characteristics defined in subpart C of Part 261, Volume 40, Code of Federal Regulations is, by definition, a hazardous waste.
A solid waste is considered to be hazardous waste if it is listed, or it exhibits characteristics of either ignitability, corrosivity, reactivity, or toxicity as determined by the Toxicity Characteristic Leaching Procedure (TCLP) (USEPA Method 1311). The test is aimed at identifying the tendency of wastes to generate a leachate with concentrations of contaminants greater than the values listed in Appendix II of the Code of Federal Regulations, Part 261.24, page 406, revised Jul. 1, 1988. For instance, if concentrations of leachable, mobile lead are found to be greater than 5 milligrams per liter, the material is considered characteristically toxic for lead and hence hazardous with respect to lead content. Such characteristically toxic wastes require treatment to comply with the USEPA regulations defining the treatment standards for the respective contaminant of concern.
Any solid waste that contains leachable TCLP constituent levels in excess of those values listed in the above referenced Appendix II is considered characteristically toxic and hence hazardous. Such hazardous waste must be treated with the appropriate Best Demonstrated Available Technologies (BDAT) and/or with an alternative technology to decharacterize the waste for heavy metal toxicity. In other words, treatment of the heavy metal-bearing solid waste with a BDAT for decreasing TCLP heavy metal to a level below the treatment standard for that metal is required before land disposal is permitted. Land disposal methods include waste staging on a land surface, placing waste into a landfill, using surface impoundment techniques, waste piling, disposing of waste in injection wells or land treatment facilities (land farming), or impounding the waste in salt domes, salt bed formations, underground mines or caves, and bunkering the waste in concrete vaults. For example, land disposal restrictions ban treated wastes with TCLP levels greater than 5 mg/l of lead in the leachate although TCLP levels other than 5 mg/l may be acceptable for other waste metals contemplated herein. Such characteristic heavy metal toxic wastes must be treated with a cost effective and practical technology that is commercially available and that provides substantial treatment, and that beneficially results in a decrease in risk to human health and the environment.
For the sake of convenience, lead is uniformly referred to in practicing the present invention. However, it must be understood that this invention contemplates, and is thus useful in the immobilization of, other cationic metals such as barium, beryllium, chrome III, cobalt, copper, nickel, silver, and zinc.
Further, although reference is repeatedly made to soil as the medium being remediated in practicing the present invention, it must be understood that various other carriers of mobile cationic metals are contemplated. Such carriers include dredge spoils, ash from, for example, incinerators, slag from, for example, steel plants, demolition materials such as concrete and brick, sludges, drilling mud and grit from, for example, sandblasting operations.
Various conventional methods have been tried to remove leachable, mobile lead from soils and solid waste materials. Those methods include washing, leaching and extracting the lead. According to conventional practice, contaminated soil or solid waste material is excavated from the ground for processing and/or washing. During washing, the contaminated material is immersed or supersaturated in water or other specified solutions while it is being agitated. Removal of lead from contaminated soils and solid wastes by leaching, extraction and/or washing procedures is extremely expensive and in many cases is cost-prohibitive because this method, generates vast quantities of lead-toxic wastewater which requires further treatment and disposal.
Other methods have included those classified as "encapsulation". The lead bearing soil is mixed with asphaltic materials and compacted into a monolithic structure with low permeability. This method assumes that the soil treated thusly will never be disturbed or reduced to a granular material. Soils treated in this manner when broken up and granulated for TCLP testing fail to pass treatment standards.
Further conventional techniques have involved the chemical fixation of lead in contaminated soils and solid waste by use of inorganic reagents such as cement, lime, silicates or alumino-silicates. These materials increase the pH of the soil to 12 or even higher and often raise questions about the long-term stability of the product. This concern is partially based upon the fact that the products made with these inorganic reagents usually have very poor strength characteristics. The reason for that is that these reagents rely on the quality of the aggregate for much of their strength. A good cement, for example, depends on a well-sorted consistent aggregate to meet quality specifications. When used to treat a soil that often contains a significant percentage of fine materials such as silts and clays, the resulting product may only be good for disposal. This, of course, means that the product is not suitable for commercial use. The emulsions of the present invention, by contrast, do not rely solely upon the properties of the aggregate to make a quality product. The asphalt or pitch base stock in the emulsion of this invention can convert a poorly sorted soil with a high percentage of silts and clays into a high quality road base material.
Various prior art approaches to soil remediation have been examined in the past. For example, U.S. Pat. Nos. 5,193,936 and 5,527,982 relate to methods of treating metal-bearing materials, such as contaminated soil, to stabilize leachable metal contained therein. The methods comprise the steps of mixing a metal-bearing material with a mixture which comprises a first component and a second component to form a second mixture, wherein (1) the metal-bearing material contains at least one leachable metal selected from a group, including lead, (2) the first component supplies at least one member selected from the group consisting of sulfates, halides and silicates, and (3) the second component supplies at least one phosphate ion. These patents discuss the EPA TCLP test and also demonstrate the utility of phosphoric acid, and phosphates in general, as lead binders for contaminated soil.
U.S. Pat. No. 5,536,899 discloses a method for reducing the leaching of lead from a lead bearing waste, consisting essentially of contacting the waste with a water soluble phosphate and a complexing agent containing iron, aluminum, chloride or combinations thereof. The process is practiced under alkaline or neutral pH conditions.
U.S. Pat. No. 4,913,586 discloses a method for the treatment of petroleum contaminated soil. Of note is the utilization of humic acid and lime as the protective mixture.
U.S. Pat. No. 4,260,493 teaches a process for the waste treatment of a spent metal plating solution containing copper or nickel.
U.S. Pat. No. 2,003,921 discloses a process for water-proofing and filling cavities in the ground. This patent discloses the use of asphalt-type emulsions for providing a water-proof soil lining.
U.S. Pat. No. 2,851,824 discloses methods of stabilizing and improving soils utilizing an asphalt-type component dispersed in an acidic aqueous medium.
U.S. Pat. No. 5,162,600 teaches a method of treating lead-contaminated soil to reduce the amount of mobile lead contained in the soil. The method comprises providing an agent selected from the group consisting of inorganic phosphate compounds, alone or as mixtures and contacting the agent with the soil to immobilize the lead contained therein. At Table 1, oxalic acid also was shown to have lead binding properties, but considered "too toxic" for use.
U.S. Pat. No. 5,304,703 discloses a process fi)r soil remediation to remove chemical soil contaminants such as polynuclear aromatics. The process involves introducing an emulsion consisting essentially of a nonionic surfactant material dispersed in water. Preferred nonionic surfactants include the polyoxyethylene adducts of tall oil, rosin acid, stearic acid and oleic acid. In the process, the emulsion is used to encapsulate the contaminated soil, with the surfactant having an affinity for the contaminants which are drawn into the encapsulating coating of the soil particles and subsequently removed with the encapsulating material in a later treatment step.
U.S. Pat. No. 3,552,130 discloses a method of injecting chemicals into soil to create a liquid impervious layer or zone. In one embodiment, the liquid impervious zone is formed by introducing bituminous emulsions into the soil.
U.S. Pat. No. 4,028,897 discloses a soil stabilization composition prepared by combining an oil (mineral oil, napthenic oils and crude oils were preferred) with a rubber latex material. If desired, bituminous or asphaltic materials may be included in the composition. The material is applied in the form of an emulsion including a nonionic emulsifier.
U.S. Pat. No. 4,260,493 discloses a composition and method for fixing hydrocarbon contaminants in soil. The process includes forming a cold asphalt mixture by blending with the contaminated soil asphalt roof cuttings containing glass fibers and a slow-setting "cold mix emulsion" of the type known in the trade as "SS-1" and "SS-1 H" emulsions. The resulting mixture can be used as a recycled asphalt pavement.
The processes as described above are still lacking in several areas. The cases that use emulsions are aimed at treating hydrocarbon contaminated soil and will not treat lead or other cationic metal contaminated soil below the respective TCLP standards. The other processes will treat the lead and other cationic metal contaminated soils below their respective TCLP standard but are not designed to create a recycled product. Further, many of these processes dramatically increase the volume of the material being treated, in some cases by as much as 50 percent.
What is still needed therefore is a process that can be adjusted to meet a variety of soil conditions, treat the leachable lead and other cationic metals to TCLP standards, create a recycled soil product that can be left on-site, or used elsewhere as a roadbase, engineered fill or aggregate material and does not increase the volume of the treated material by inordinate amounts.